Statistical Illuminator

ABSTRACT

The technology disclosed relates to an illumination source including numerous laser diodes. In particular, it relates to extending the duty cycle and/or reducing the frequency of component replacement by detecting failure of one or more individual laser diodes and compensating for the failure, without replacing the laser diodes. 
     The technology disclosed can be used in cases of catastrophic laser diode failure by changing the power of remaining laser diodes to restore illumination to the coherence function similar to the pre-failure illumination field. Particular aspects of the technology disclosed are described in the claims, specification and drawings.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/158,310, filed 6 Mar. 2009, which is hereby incorporated byreference.

This application is related to US patent application entitled “RotorImaging System and Method with Variable-Rate Pixel Clock”; and US patentapplication entitled “Variable Overlap Method and Device for StitchingTogether Lithographic Stripes”; and US patent application entitled“Lithographic Printing System with Placement Corrections”, all filedcontemporaneously. The related applications are incorporated byreference.

BACKGROUND OF THE INVENTION

The technology disclosed relates to an illumination source includingnumerous laser diodes. In particular, it relates to extending the dutycycle and/or reducing the frequency of component replacement bydetecting failure of one or more individual laser diodes andcompensating for the failure, without replacing the laser diodes.

The Micronic Laser development team has pioneered a variety of platformsfor microlithographic printing. An established platform for the Sigmamachine is depicted in FIG. 2. A rotor printing platform is described inrecently filed patent applications. A drum printing platform isdescribed in other patent applications.

One printing mechanism designed for these platforms uses swept beamsthat are modulated as they traverse the surface of the workpiece,applying energy as a paintbrush applies color. Another printingmechanism design freezes the motion of the workpiece with the flash andstamps two dimensional patterns on the workpiece, exposing a radiationsensitive layer in a manner similar to block printing a pattern.Printing with stamps is an intricate process that typically overlapsmultiple writing passes.

Illuminators are a major part of the operating cost of manymicrolithographic printing systems. Accordingly, the opportunity is everpresent to develop new illuminators. New illuminator designs may deliverincreased power, extended lives, failure tolerance and decreasedmaintenance.

SUMMARY OF THE INVENTION

The technology disclosed relates to an illumination source includingnumerous laser diodes. In particular, it relates to extending the dutycycle and/or reducing the frequency of component replacement bydetecting failure of one or more individual laser diodes andcompensating for the failure, without replacing the laser diodes.

The technology disclosed can be used in cases of catastrophic laserdiode failure by changing the power of remaining laser diodes to restoreillumination to the coherence function similar to the pre-failureillumination field. Particular aspects of the technology disclosed aredescribed in the claims, specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Partially coherent projection system using an SLM and relays inthe illuminator and projection paths.

FIG. 2: Generic writer or printer using a one-dimensional SLM asdisclosed.

FIG. 3: An example illuminator using an array of light sources S1-S8.

FIG. 4 a: Gaussian distribution of light from the plane of the sources.

FIG. 4 b: Depicting the coherence function with the failed laser diode.

FIG. 4 c: Depicting the solution for a failed laser diode.

FIG. 5: A flow chart depicting the iteration process for maintaining aconstant value for illumination field.

DETAILED DESCRIPTION

The following detailed description is made with reference to thefigures. Preferred embodiments are described to illustrate thetechnology disclosed, not to limit its scope, which is defined by theclaims. Those of ordinary skill in the art will recognize a variety ofequivalent variations on the description that follows.

The technology disclosed uses a one or two dimensional array of laserdiodes with individually controlled power feeds and radiation outputs asan illumination source. Based on our analysis, we believe that 15 laserdiodes is a good minimum number to permit the system to continueoperation after catastrophic failure of one or more laser diodes, whilecontinuing to satisfy a selected coherence function and afford aprinting fidelity. Our analysis demonstrates that seven or eight laserdiodes is too few to permit reduction of output from one of theremaining good laser diodes, to reestablish symmetry. By catastrophicfailure, we mean that one or more laser diodes suffers a reduction inoutput to less than or equal to 20% of its initial output. By continuingto operate, we mean that the array of laser diodes can be used withoutreplacing the failed laser diode. Coupled to an array of 15 or morelaser diodes, we describe a detection and recovery method device toavoid the inconvenience of interrupted production and increase the timebetween replacement of illumination elements or illumination sources.

FIG. 3 depicts an example illuminator using an array of light sourcesS1-S8. The number of sources can vary from case to case, e.g. dependingon the number of sources necessary to reach the desired total opticalpower. The sources may be laser diodes, e.g. with wavelengthapproximately 405, 375, or 360 nm. The number of sources needs to belarger or much larger than shown, at least 15 laser diodes rangingupward to 30, 60, 120, or 200 sources. The sources may be discrete,mounted in mechanic modules or be part of laser bars. The sources areincoherent to each other, e.g. by being selected to have a slightlydifferent wavelength. The light from each source is collimated to fallon the SLM from one direction. The sources together create a partiallycoherent illumination on the SLM, which is beneficial for resolution andimage contrast in an image subsequently formed of the illuminated SLM.The partial coherence may be described by a coherence function, wellknown to the skilled optical engineer and described in Born & Wolf:“Principles of optics” and other textbooks. The coherence factor can becalculated by means of the Zernike-van Cittert formula found in thetextbooks, relating the coherence function to the angular spread of thelight impinging on it.

The coherence function may have an approximately Gaussian or sin(x)/xshape. The exact shape is a compromise between desired imagingproperties and technical limitation of the illuminator. An approximatelyGaussian shape assumes an approximate Gaussian distribution of lightfrom the plane of the sources. An example is shown in FIG. 4 a. Thefilled dots show the actual power from each laser diode and the opendots the maximum allowed power from the same laser diodes. The laserdiodes are shown with an even spatial distribution, but it is possibleto use varying distances, thereby creating the desired powerdistribution with few laser diodes, and/or most laser diodes operatingcloser to their maximum power. The graph to the right in FIG. 4 a showsthe resulting coherence function.

The laser diodes have a limited life and the cost of the laser diodes isa large part of the cost of ownership of the disclosed laser writer.Typically the laser diodes fade slowly during life, but catastrophicfailure also happens. Such a failure is shown in the example in FIG. 4b. Having a laser diode fail can cause a number of problems.

First, if the deterioration of the coherence function is large enough toaffect image properties, the writing system may need to be taken downfor repair immediately, upsetting production planning. If the repaircannot be affected immediately, the system may be down for hours orlonger until a skilled service person with the proper spare partsarrives.

Second, if the light sources, e.g. laser diodes, are mounted in modulesor form part of the same array component, one failed source meanschanging a whole module or array, incurring higher replacement costs.

In addition, having a tunable fault-tolerant scheme may allow laserdiodes or laser diode arrays with less tight specifications to be used.To be able to run the system with laser diode arrays with some laserdiodes performing out of spec may save cost and in some cases even makeit possible to use lasers which cannot be reproducibly produced, e.g. atshorter wavelengths.

FIG. 4 b shows the coherence function with the failed laser diode in theleft figure. The figure shows the actual coherence function (“Act”) andthe intended one (“Ref”) and the difference magnified ten times(“10*Diff”). The horizontal scale may, in some embodiments, be equal tothe number of pixels in the SLM. The difference between intended andactual coherence function translates to errors in the image, e.g. in thebalance between the size of small and large features. The figure alsoshows the phase angle of the complex coherence function in milliradians.A tilted phase angle from mirror to mirror is the same as an apparenttilt in the illumination and will give problems with the landing angleof the light in the image, i.e. the image gets a displacement sidewayswhen focus is changed.

The problem of failing laser diodes or laser diodes out of specificationmay be solved as shown in FIG. 4 c. The power of some of the other laserdiodes has been changed to restore the coherence function to be moresimilar to the intended one. To avoid problems with the landing anglethe distribution of power is made symmetrical by the lowering of thepower of the laser diode symmetrical to the failed one on the other sideof the optical axis. Doing this improves landing angle, but amplifiesother image errors, like the large-small balance. Laser diodes close tothe laser diodes with low power are adjusted to a higher power. In ageneral procedure each may have a different limit and the distributionmay be more complicated than shown in the figure.

The adjustment of the power to the laser diodes may be doneautomatically by calculation of the coherence function or even theproperties of the image and finding, e.g. by iteration, laser diodecurrents that minimize the resulting errors.

Another possibility is to specify momenta of different orders for thelight intensity and bringing the momenta within bounds by modifying thedrive currents to the laser diodes. In some cases it may not be possibleto recreate the desired momenta, coherence functions, or imageproperties at the same total power. In those cases, a lower power may beset and the writing speed of the laser writer reduced, in order to keepit running until a repair can be done. Likewise it may be possible torun some laser diodes beyond their safe power levels in order to keepthe system running until a repair can take place, thereby eating intothe lifetime of the laser diodes slightly, but avoiding unscheduleddowntime.

The light source may be measured constantly or at short regularintervals using an array of detectors or a camera. The image may bebrought to the camera by means of a beam sampling mirror or gratingalways present in the system.

The tuning of the light source currents may be automated in thebackground by the following procedure. FIG. 5 depicts an iterationprocess used to calculate a coherence function and solve for a newpossible distribution of laser diode power. First, measure the lightdistribution 513 in the source plane. Second, calculate a qualityfunction 515, which may be first, second, etc. or momenta of the lightdistribution; or the coherence function from Zernike-van Cittert, or theimage of one or more features in the image. Next, solve the derivedcoherence function to determine a new possible distribution of laserdiode power 517 using the quality index. If the quality index is withinallowed bounds 519 then check to see if it is close to the boundarylimit 521. If yes, then alert the operator 519 to schedule the repair.However, if the quality index is not within the allowed bounds thenalert the operator 539 that the system is out of bounds.

In the following paragraphs, we describe systems that use 2D and 1D SLMsthat require illumination services.

A generic projection system is illustrated by FIG. 1. It has an object1, which can be a mask or one or several SLMs, and a workpiece 2, e.g. amask blank, a wafer or a display workpiece 2 device. Between them is aprojection system 3 that propels 4 onto the image 5. The object isilluminated by an illuminator 6. The projection system consists of oneor several lenses (shown) or curved mirrors. The NA of the projectionsystem is determined by the size of the pupil 8. The illuminator 6includes a light source 17 illuminating the illumination aperture 19.Field lenses 10 and 11 are shown but the presence of field lenses is notessential for the function. The imaging properties are determined by thesize and intensity variation inside the illuminator aperture 9 inrelation to the size of the pupil 8.

The basic projection system in 1 a can be realized in many equivalentforms, e.g. with a reflecting object as shown in FIG. 1. The imagingpower of the optical system can be refractive, diffractive or residingin curved mirrors. The reflected image can be illuminated through a beamsplitter 12 or at an off-axis angle. The wavelength can be ultravioletor extending into the soft x-ray (EUV) range. The light source can becontinuous or pulsed: visible, a discharge lamp, one or several lasersources or a plasma source. The object can be a mask in transmission orreflection or an SLM. The SLM can be binary or analog; for examplemicromechanical, using LCD modulators, or using olectrooptical,magnetooptical, electroabsorbtive, electrowetting, acoustooptic,photoplastic or other physical effects to modulate the beam.

The Sigma7300 mask writer made by Micronic Laser Systems AB furtherincludes as an Excimer laser 17, a homogenizer 18, and relay lenses 13forming an intermediate image 14 between the SLM and the final lens. Thepupil of the final lens is normally located inside the enclosure of thefinal lens and difficult to access, but in FIG. 1 there is an equivalentlocation 15 in the relay. The smallest of the relay and lens pupils willact as the system stop. There is also a relay in the illuminatorproviding multiple equivalent planes for insertion of stops and baffles.In some implemantations, the Sigma7300 has a catadioptric lens with acentral obscuration of approximately 16% of the open radius in theprojection pupil.

FIG. 2 is a rendering of the Sigma system, using a two-dimensional SLMas disclosed. A light source 205 (arc lamp, gas discharge, laser, arrayof lasers, laser plasma, LED, array of LEDs etc.) illuminates aone-dimensional SLM 204. The reflected (or transmitted in the generalcase) radiation is projected as a line segment 203 on a workpiece 201.The data driving the SLM changes as the workpiece is scanned 207 tobuild up an exposed image. A strongly anamorphic optical system 206concentrates energy from multiple mirrors in a column (or row) to pointin the image and the entire two-dimensional illuminated array forms anarrow line segment 203 that is swept across the workpiece. In onedimension, the anamorphic optics demagnify the illuminated area, forinstance, by 2× to 5×, so the a 60 millimeter wide SLM would image ontoa line segment 30 to 12 mm long. Along the short dimension, theanamorphic optics strongly demagnify the column of mirrors to focus ontoa narrow area such as 3 microns wide, i.e. essentially a single resolvedline. Alternatively, the area could be 1 or 5 microns wide or less than10 microns wide. Focus onto a 3 micron wide area could involve an 80×demagnification, from approximately 240 microns to 3 microns. Theanamorphic optical path demagnifies the row of mirrors to an extent thatindividual mirrors are combined and not resolved at the image plane.

Some Particular Embodiments

The technology disclosed may be practiced as a method or device adaptedto practice the method. The technology disclosed may be an article ofmanufacture such as media impressed with logic to carry outcomputer-assisted method or program instructions that can be combinedwith hardware to produce a computer-assisted device.

One embodiment is a method of extending the life of an illuminationsource upon catastrophic failure of one or more illumination elementsamong 15 or more elements. The method includes operating an illuminatorthat combines radiation output from 15 or more illumination elements.The illuminator distributes initial power to the elements that producesinitial radiation output levels from the elements. The illuminator alsocombines the initial radiation output levels to produce an overallillumination field from the illuminator that satisfies a qualityfunction. Next, there is detection of failure of a first illuminationelement that reduces output from the first element to less than 20percent of its initial output level. The power distribution to andoutput from one or more non-failing illumination elements is reduced torestore symmetry in the overall illumination field. The powerdistribution to and output from at least some of the illuminationelements is increased to restore quality of the overall illuminationfield, as measured by the quality function.

In alternate embodiments, the illuminator combines radiation from 15 upto 200 illumination elements. The illumination elements can also havevarying spatial distribution.

One aspect of the technology disclosed, applicable to any of theembodiments above, is expressing said quality function as anapproximately Gaussian distribution. Alternately, the quality functioncan also be expressed as an approximately sin(x)/x distribution.

Another aspect of the technology disclosed is automatically detecting,reading and increasing power distribution.

In another embodiment, the illuminator operates with the 15 or moreillumination elements after the first illumination element fails,without replacing the failed first illumination element.

In yet another embodiment, failure of a second illumination element isdetected, applying the reducing and increasing steps to compensate forthe failure of the second illumination element, and continuing tooperate the illuminator with the 15 or more illumination elements afterthe first and second illumination elements have failed, withoutreplacing the first or second illumination elements.

Any of the methods described above or aspects of the methods may beembodied in a self correcting illuminator system. The system includes anilluminator that includes 15 or more illumination elements and opticsthat combine radiation output from the illumination elements, a powersupply coupled to the illumination elements that distributes power tothe illumination elements, sensors optically coupled to the radiationoutput, a controller coupled to the sensors and controlling the powersupply, the controller including program instructions that set aninitial power level for the illumination elements, wherein initialoutput levels from the illumination elements produce an overallillumination field from the illuminator that satisfies a qualityfunction. The controller also detects failure of a first illuminationelement that reduces output from the first element to less than 20percent of its initial output level. The controller is furtherresponsive to the detected failure, reduce power distribution to andoutput from one or more non-failing illumination elements to restoresymmetry in the overall illumination field and also responsive to thedetected failure, increase power distribution to and output from atleast some of the illumination elements to restore quality of theoverall illumination field, as measured by the quality function.

One aspect of the technology disclosed is illumination elements havingeven spatial distribution. Alternately, the illumination elements canalso have varying spatial distribution.

Another aspect of the technology disclosed is expressing said qualityfunction as an approximately Gaussian distribution. Alternately, thequality function can also be expressed as an approximately sin(x)/xdistribution.

While the technology is disclosed by reference to the preferredembodiments and examples detailed above, it is understood that theseexamples are intended in an illustrative rather than in a limitingsense. Computer-assisted processing is implicated in the describedembodiments, implementations and features. Accordingly, the disclosedtechnology may be embodied in methods for reading or writing a workpieceusing at least one optical arm that sweeps an arc over the workpiece,systems including logic and resources to carry out reading or writing aworkpiece using at least one optical arm that sweeps an arc over theworkpiece, systems that take advantage of computer-assisted control forreading or writing a workpiece using at least one optical arm thatsweeps an arc over the workpiece, media impressed with logic to carryout, data streams impressed with logic to carry out reading or writing aworkpiece using at least one optical arm that sweeps an arc over theworkpiece, or computer-accessible services that carry outcomputer-assisted reading or writing a workpiece using at least oneoptical arm that sweeps an arc over the workpiece. It is contemplatedthat modifications and combinations will readily occur to those skilledin the art, which modifications and combinations will be within thespirit of the disclosed technology and the scope of the followingclaims.

1. A method of extending the life of an illumination source uponcatastrophic failure of one or more illumination elements among 15 ormore elements, the method including: operating an illuminator thatcombines radiation output from 15 or more illumination elements,including distributing initial power to the elements that producesinitial radiation output levels from the elements; and combining theinitial radiation output levels to produce an overall illumination fieldfrom the illuminator that satisfies a quality function; detectingfailure of a first illumination element that reduces output from thefirst element to less than 20 percent of its initial output level;reducing power distribution to and output from one or more non-failingillumination elements to restore symmetry in the overall illuminationfield; increasing power distribution to and output from at least some ofthe illumination elements to restore quality of the overall illuminationfield, as measured by the quality function.
 2. The method of claim 1,further including the illuminator combining radiation from up to 200illumination elements.
 3. The method of claim 1, wherein theillumination elements have even spatial distribution.
 4. The method ofclaim 1, wherein the illumination elements having varying spatialdistribution.
 5. The method of claim 1 further including expressing saidquality function as an approximately Gaussian distribution.
 6. Themethod of claim 1, further including expressing said quality function asan approximately sin(x)/x distribution.
 7. The method of claim 1,wherein said detecting, reading and increasing power distribution can bedone automatically.
 8. Method of claim 1, further including operatingthe illuminator with the 15 or more illumination elements after thefirst illumination element has failed without replacing the firstillumination element.
 9. The method of claim 1, further includingdetecting failure of a second illumination element, applying thereducing an increasing steps to compensate for the failure of the secondillumination element, and continuing to operate the illuminator with the15 or more illumination elements after the first and second illuminationelements have failed, without replacing the first or second illuminationelements.
 10. A self-correcting illuminator system including: anilluminator that includes 15 or more illumination elements and opticsthat combine radiation output from the illumination elements; a powersupply coupled to the illumination elements that distributes power tothe illumination elements; sensors optically coupled to the radiationoutput; a controller coupled to the sensors and controlling the powersupply, the controller including program instructions that set aninitial power level for the illumination elements, wherein initialoutput levels from the illumination elements produce an overallillumination field from the illuminator that satisfies a qualityfunction; detect failure of a first illumination element that reducesoutput from the first element to less than 20 percent of its initialoutput level; responsive to the detected failure, reduce powerdistribution to and output from one or more non-failing illuminationelements to restore symmetry in the overall illumination field; andfurther responsive to the detected failure, increase power distributionto and output from at least some of the illumination elements to restorequality of the overall illumination field, as measured by the qualityfunction.
 11. The system of claim 10, wherein the quality function is anapproximately Gaussian distribution.
 12. The system of claim 10, whereinthe quality function is an approximately sin(x)/x distribution.
 13. Thesystem of claim 10 wherein the illumination elements have even spatialdistribution.
 14. The system of claim 10 wherein the illuminationelements have varying spatial distribution.